Methods for the electroreduction of carbon dioxide to value added chemicals

ABSTRACT

The present disclosure provides a method of electroreducing carbon dioxide (CO 2 ). The method of electroreducing carbon dioxide may include feeding a first stream comprising carbon dioxide into a chamber through a chamber inlet, the chamber containing a gas diffusion cathode and a gas diffusion anode; feeding a second stream comprising glycerol or glucose into the chamber, the second stream having a pH of 12 to 14; and applying an electrical potential between the gas diffusion anode and the gas diffusion cathode to reduce the carbon dioxide and oxidize the glycerol or glucose.

RELATED APPLICATIONS

The present patent document claims the benefit of priority under 35U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/546,044,filed on Aug. 16, 2017, which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present disclosure is related generally to the electroreduction ofcarbon dioxide and more particularly to a method of electrochemicallyreducing carbon dioxide and oxidizing glycerol or glucose.

BACKGROUND

The global atmospheric carbon dioxide (CO₂) concentrations have been ona constant rise in the past few decades, with the daily average valuecrossing and staying above the 400-ppm mark in 2016, for the first timein recorded human history. The rise in CO₂ levels has been correlated tothe increase in mean global temperature anomalies (global warming).Thus, developing cost effective technologies that can mitigate, capture,or utilize the excess anthropogenic CO₂ emissions remains a grandchallenge of the 21^(st) century. To limit excess anthropogenic carbondioxide (CO₂) emissions (˜4GtC yr⁻¹), and achieve the 2° C. target setforth in the Paris climate change agreement, a portfolio oftechnologies, such as (i) transitioning from fossil fuels to renewableenergy (wind, solar, biofuels, etc.); (ii) improving the energyefficiency of vehicles and buildings; and (iii) CO₂ capture andsequestration, need to be implemented together. However, for a majorityof these solutions, the associated costs and impact on economic growthis high, resulting in slow global adoption. An alternative to mitigatingCO₂ emissions could be the utilization of CO₂ as a resource to producevalue added chemicals, such as formate/formic acid (HCOO⁻/HCOOH), carbonmonoxide (CO), methane (CH₄), methanol (CH₃OH), ethylene (C₂H₄), andethanol (C₂H₅OH) via an electrochemical (i.e., electroreduction)approach, that are currently manufactured on the industrial scale usingcarbon intensive fossil fuel methods.

The dominant design for state of the art electrochemical CO₂ conversionprocesses consists of a cathodic CO₂ reduction reaction (CO₂RR) coupledto an anodic oxygen evolution reaction (OER). The electrochemical systemis characterized by a standard cell potential (E⁰ _(cell)) thatrepresents the minimum thermodynamic energy required to drive thereaction. Thermodynamic analysis of these two reactions shows that ˜90%of the overall energy (hence, cell potential) requirements comes fromthe OER. As a result, there is a need to think beyond OER and identifyother oxidation reactions (with a lower thermodynamic energy barrier)that can lower E⁰ _(cell).

Lowering E⁰ _(cell) represents one of the most important factors inmaking electrochemical conversion of CO₂ economically viable.Additionally, for a practical implementation of the process, it would beadvantageous to drive the reaction using grid electricity (comprisingmainly of fossil fuel resources) and remain carbon neutral or negative.

BRIEF SUMMARY

According to one embodiment, a method of electroreducing carbon dioxidecomprises: feeding a first stream comprising carbon dioxide into achamber through a chamber inlet, the chamber containing a gas diffusioncathode and a gas diffusion anode; feeding a second stream comprisingglycerol or glucose into the chamber, the second stream having a pH of12 to 14; and applying an electrical potential between the gas diffusionanode and the gas diffusion cathode to reduce the carbon dioxide andoxidize the glycerol or glucose.

According to another embodiment, a method of electroreducing carbondioxide comprises: feeding a gas stream comprising carbon dioxide intochamber divided into a cathode compartment and an anode compartment byan ion permeable membrane, the anode compartment containing a gasdiffusion anode and the cathode compartment containing a gas diffusioncathode, the gas diffusion cathode dividing the cathode compartment intoa gaseous region and a liquid region, wherein the gas stream is fed intothe gaseous region; feeding a catholyte stream into the liquid region ofthe cathode compartment; feeding an anolyte stream comprising glycerolinto the anode compartment to contact the gas diffusion anode; andapplying an electrical potential between the gas diffusion anode and thegas diffusion cathode to reduce the carbon dioxide to a reductionproduct and oxidize the glycerol to an oxidation product, wherein anonset cell potential for formation of the reduction product is from −1.5V to −0.5 V.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription that follows may be better understood. Additional featuresand advantages of the disclosure will be described hereinafter that formthe subject of the claims of this application. It should be appreciatedby those skilled in the art that the conception and the specificembodiments disclosed may be readily utilized as a basis for modifyingor designing other embodiments for carrying out the same purposes of thepresent disclosure. It should also be realized by those skilled in theart that such equivalent embodiments do not depart from the spirit andscope of the disclosure as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of the invention is hereafter described withspecific reference being made to the drawings in which:

FIG. 1 is a block diagram of a system according to an embodiment of thepresent disclosure;

FIG. 2 is a block diagram of a system according to an embodiment of thepresent disclosure;

FIG. 3 is a block diagram of a system according to an embodiment of thepresent disclosure;

FIG. 4A shows partial current density for CO (j_(CO)) as a function ofthe cell potential for the electroreduction of CO₂ to CO coupled to O₂evolution, glycerol electrooxidation, or glucose electrooxidation at theanode;

FIG. 4B shows individual electrode potential as a function of the totalcurrent density (j_(Total)) for the electroreduction of CO₂ to COcoupled to O₂ evolution, glycerol electrooxidation, or glucoseelectrooxidation at the anode;

FIG. 5 shows variation in the cell potential and Faradaic efficiency forCO production as a function of time; and

FIGS. 6A-6C show partial current density for 6A, HCOO⁻ (j_(HCOO—)) 6B,C₂H₄ (j_(C) ₂ _(H) ₄ ) and 6C, C₂H₅OH (j_(C) ₂ _(H) ₅ _(OH)) as afunction of the cell potential for the electroreduction of CO₂.

DETAILED DESCRIPTION

Various embodiments are described below. The relationship andfunctioning of the various elements of the embodiments may better beunderstood by reference to the following detailed description. Theembodiments, however, are not limited to those illustrated below. Incertain instances details may have been omitted that are not necessaryfor an understanding of embodiments disclosed herein.

The present disclosure provides a method of electrochemically convertingCO₂, glycerol, and glucose to value added carbon chemical feedstocks. Afeature of this method entails the use of glycerol or glucoseelectrooxidation as the anodic reaction instead of the traditionallyused OER for CO₂ electroreduction systems. According to the methoddisclosed herein, the required cell potentials can be lowered, therebyreducing the electrical energy requirements by up to 53%. Some benefitsmay include (i) the process lowers electrical energy requirement incomparison to the state of the art CO₂ electroreduction systems leadingto lower operating cost; (ii) the process is close to economicviability; (iii) the process could potentially be carbonneutral/negative even when using grid electricity to drive the process;and (iv) the process produces value added carbon chemical feedstocks.

The methods disclosed herein can be performed in electrochemical cellshaving different configurations. For example, FIGS. 1-3 depictembodiments of an electrochemical cell having different positions of theion permeable membrane.

Referring to FIG. 1, a method of electroreducing carbon dioxide isprovided. The method can include feeding a first stream 112 comprisingcarbon dioxide into a chamber 102 through a chamber inlet. The chamber102 contains a gas diffusion cathode 103 and a gas diffusion anode 104.The method includes feeding a second stream 113 into the chamber 102.The second stream 113 comprises glycerol or glucose and has a pH of 12to 14. The method includes applying an electrical potential between thegas diffusion anode 104 and the gas diffusion cathode 103 to reduce thecarbon dioxide and oxidize the glycerol or glucose. The gas diffusioncathode 103 may divide the chamber 102 into a gaseous region 106 and aliquid region 107.

The electrochemical cell 100 can include a CO₂ source 101 from which thefirst stream 112 is fed into a chamber 102. A glycerol or glucose source110 can be fed into the chamber 102. An electricity source 109 isconnected to the gas diffusion cathode 103 and the gas diffusion anode104 for applying an electrical potential between the electrodes toreduce the carbon dioxide and oxidize the glycerol or glucose.

Gaseous reduction products 110 can be withdrawn from an outlet of thecompartment in a gaseous product stream 114 to be purifed. Liquid phasereduction products and oxidation products 111 can be withdrawn from thechamber 102 in a liquid product stream 115 to be purified.

The liquid phase products 111 include certain reduction products of CO₂such as, for example, ethanol or formate and oxidation products ofglycerol or glucose. Without an ion permeable membrane the liquidproducts of the reactions at the cathode and anode mix and are withdrawntogether for purification.

The method is not limited to the cell configuration shown in FIG. 1.Referring to FIG. 2, an electrochemical cell 200 may include a chamber102 divided into a cathode compartment 202 and an anode compartment 206by an ion permeable membrane 201. The ion permeable membrane 201 allowsthe passage of electrons from one compartment to the other whilepreventing transport of certain molecules.

Gaseous reduction products 110, liquid phase reduction products 2033,and oxidation products 204 can be withdrawn from the chamber 102 andpurified. Liquid reduction products 203 can be withdrawn from the liquidregion 107 in a liquid reduction product stream 205.

In another embodiment, a method of electroreducing carbon dioxide isprovided. The method can include feeding a gas stream comprising carbondioxide into chamber divided into a cathode compartment and an anodecompartment by an ion permeable membrane. The anode compartment containsa gas diffusion anode and the cathode compartment contains a gasdiffusion cathode. The gas diffusion cathode divides the cathodecompartment into a gaseous region and a liquid region, where the gasstream is fed into the gaseous region. The method includes feeding acatholyte stream into the liquid region of the cathode compartment;feeding an anolyte stream comprising glycerol into the anode compartmentto contact the gas diffusion anode; and applying an electrical potentialbetween the gas diffusion anode and the gas diffusion cathode to reducethe carbon dioxide to a reduction product and oxidize the glycerol to anoxidation product. The onset cell potential for formation of thereduction product can be from −1.5 V to −0.5 V.

Referring to FIG. 3, an electrochemical cell 300 may include a chamber102 where an ion permeable membrane 301 may be in contact with the gasdiffusion cathode 103. The ion permeable membrane 301 is positionedadjacent to and in contact with the gas diffusion cathode 103. Gaseousreduction products 110 and liquid phase products 302 can be withdrawnfrom the chamber 102 and purified.

The electrochemical cell of the present disclosure can be flowelectrolyzer that is a modified version of the electrochemical celldisclosed in U.S. Pat. No. 7,635,530, which is incorporated by referencein its entirety. In some embodiments, the chamber is defined inside aflow electrolyzer.

The methods disclosed herein can include withdrawing a liquid productstream from an outlet of the chamber. The liquid product stream caninclude oxidation products of glycerol or glucose or reduction productsof carbon dioxide. The method can include withdrawing a gaseous productstream that includes a reduction products of carbon dioxide. Gaseousreduction products of carbon dioxide can be selected from carbonmonoxide, ethylene, methane, and any combination thereof. Table 1 showsmajor products of the reduction of CO₂ associated with metal catalysts.

TABLE 1 Cathode Reaction Catalyst Major Product CO₂ electroreduction Au,Ag, Zn, Pd Carbon monoxide (gas) Sn, Pb, Hg, In Formate (liquid) CuCarbon monoxide (gas), ethylene (gas), ethanol (liquid)

In some embodiments, the carbon dioxide is reduced to carbon monoxide.The partial current density for carbon monoxide can be anywhere from 15mA cm⁻² to 350 mA cm⁻² at a cell potential of −1.0 V to −2.5 V. In someembodiments, the partial current density for carbon monoxide can beanywhere from 15 mA cm⁻² to 100 mA cm⁻² at a cell potential of −1.0 V to−1.5 V.

In some embodiments, the carbon dioxide is reduced to ethanol, formate,or any combination thereof.

In some embodiments, the carbon dioxide is reduced to a reductionproduct of carbon dioxide and an onset cell potential for formation ofthe reduction product of carbon dioxide is from −1.5 V to −0.5 V.

Table 2 shows major products of the oxidation of glycerol and glucose.

TABLE 2 Anode Reaction Catalyst Major Product Glycerol Pt blackGlyceraldehyde (liquid), electrooxidation formate (liquid), lactate(liquid) Glucose Pt black Gluconate (liquid) electrooxidation

In one embodiment, the method can include additional purification stepsto produce substantially pure streams of the reduction and oxidationproducts. The method can include purifying the liquid product stream toobtain a substantially pure stream of the oxidation product of glycerolor glucose or the reduction product of carbon dioxide. The method canalso include purifying the gaseous product stream to obtain asubstantially pure stream of the reduction product of carbon dioxide.

The oxidation and reduction products can be purified using knowntechniques. For example, carbon monoxide and other gaseous products canbe separated from the gaseous product stream using pressure swingadsorption. Ethanol can be separated from the liquid product streamusing distillation.

The second stream, according to the disclosure, can be an electrolytethat contacts both the anode and the cathode when an ion permeablemembrane is not present. When an ion permeable membrane is present thatdivides the chamber into an anode compartment and a cathode compartment,the second stream can include an anolyte stream and a catholyte stream.The anolyte stream is fed into the anode compartment and the catholytestream is fed into the cathode compartment.

The second stream can include an alkali metal hydroxide or carbonatesalts of Na⁺, K⁺, Rb⁺, or Cs⁺. The concentration of the alkali metalhydroxide or carbonate salt in the second stream can be from 0.5 M to2.5 M. In some embodiments, the concentration of the alkali metalhydroxide or carbonate salt in the second stream is 2.0 M. Preferably,the second stream contains 2.0 M of potassium hydroxide.

The pH of the second stream can range from 12 to 14. For example, the pHcan be from 12.5 to 14, 13 to 14, or greater than 12. In someembodiments, the pH of the catholyte and the anolyte can be from 12 to14, 12.5 to 14, 13 to 14, or greater than 12.

Operating the electrochemical reactions at a pH above 12 enables greaterselectivity and selectivity while also lowering overpotential for CO₂reduction. The overpotential is the difference between the theoreticalcell potential and the observed cell potential. In some embodiments, theoverpotential for CO₂ reduction can be from −0.1 V to −1.0 V.

In some embodiments, the catholyte can include an alkali metal hydroxidesalt of Na⁺, K⁺, Rb⁺, or Cs⁺. The catholyte can include 0.5 to 2.5 M ofan alkali metal hydroxide salt. Preferably, the concentration of thealkali metal hydroxide salt in the catholyte is 2.0 M.

In some embodiments, the anolyte can include an alkali metal hydroxideor carbonate salt of Na⁺, K⁺, Rb⁺, or Cs⁺. The anolyte can include 0.5to 2.5 M of an alkali metal hydroxide or carbonate salt. Preferably, theconcentration of the alkali metal hydroxide salt in the anolyte is 2.0M.

The second stream or anolyte stream can contain from 0.5 M to 3.0 M ofglycerol or glucose. In some embodiments, the concentration of glycerolor glucose is 2.0 M.

The gas diffusion cathode, according to the present disclosure, caninclude an electrocatalytic cathode coating configured for reducingcarbon dioxide. The electrocatalytic cathode coating can include a metalselected from silver, gold, zinc, palladium, tin, lead, mercury, indium,copper, and any combination thereof.

The gas diffusion anode, according to the present disclosure, caninclude an electrocatalytic anode coating configured for oxidizingglycerol. The electrocatalytic anode coating comprises a platinum blackcatalyst.

The methods electrochemically reducing CO₂ to platform chemicals such ascarbon monoxide, formate, ethylene, and ethanol on a silver, gold, zinc,palladium, tin, lead, mercury, indium, or copper catalyst, whilesimultaneously performing an electrochemical oxidation of glycerol,glucose to value added products such as glyceraldehyde, formate,lactate, gluconate on a platinum black catalyst. The CO₂electroreduction catalysts may be in a nanoparticle form. In a relatedembodiment of the method, the electroreduction of carbon monoxide toethylene and ethanol on copper nanoparticle catalyst can be coupled tothe electrochemical oxidation of glycerol or glucose to glyceraldehyde,formate, lactate, gluconate on a platinum black catalyst. All theelectrochemical reactions can be carried out in a gas diffusionelectrode based flow electrolyzer.

Rationale for Method

The electroreduction of CO₂ could become carbon neutral and/or negativeeven with grid electricity. The current share of low carbon renewablesin the U.S. electricity grid is low (13%), and projected not to exceed30% by 2040. Being able to drive CO₂ electroreduction using gridelectricity instead of pure renewables and still be carbon neutraland/or negative could be an ideal scenario, as the process can beintegrated into the existing infrastructure.

Thus, utilizing anode reactions with energy requirements lower than OERcould be a strategy for radically lowering the energy requirements forCO₂ electroreduction. The anodic oxidation of glycerol, a cheapbyproduct of industrial biodiesel and soap production, coupled to thecathodic reduction of CO₂ (i.e., co-electrolysis of CO₂ and glycerol)can lower the energy requirements compared to OER. Co-electrolysis ofCO₂ and glycerol lowers the CO₂ electroreduction cell potential by about0.85 V, resulting in a reduction in electricity consumption by up to53%.

Glycerol can be obtained on an industrial scale as a waste byproducteither from the biodiesel industry or the soap industry. Due to theincrease in the global biodiesel production, glycerol supply (2.4 MT in2007) far exceeds demand (1 MT in 2007), and has resulted in a drop inthe glycerol prices to values as low as $0.05 lb⁻¹. On the other hand,glucose can be obtained in large amounts as a renewable energy sourcefrom waste agricultural biomass or energy crops such as corn.

The electrooxidation of high volume building block chemicals such asglycerol, biomass derived glucose, or even CH₄ (large natural gasreserves, otherwise flared off-gas at oil fields), could satisfy theprocess design rules for suitable anode reactions. Table 3 shows thecalculated ΔG⁰ _(reaction) and |E⁰ _(cell)| values for selectcombinations of CO₂ electroreduction with glycerol, glucose, and CH₄electrooxidation. The values suggest that a significant lowering of |E⁰_(cell)| and hence, electricity requirements can be realized by movingaway from the anodic OER.

In certain cases, the electroreduction of CO₂ to CH₃OH, C₂H₄, or C₂H₅OHon the cathode with the electrooxidation of glucose to gluconic acid onthe anode the process becomes spontaneous (ΔG⁰ _(reaction)<0), i.e.,behaves like a fuel cell, and can thus in principle, be used for thesimultaneous production of electricity and carbon chemicals.

The Gibb's free energy of reaction can be calculated using the followingformula:

ΔG ⁰ _(reaction) =Σv _(product) *ΔG _(f) _(product) ⁰ −Σv _(reactant)*ΔG _(f) _(reactant) ⁰

where v=stoichiometric coefficient and ΔG⁰ _(f)=Gibb's free energy offormation. |E⁰ _(cell)|=|−ΔG⁰/z*F| where z=number of electronstransferred and F=Faraday's constant=96485 C mol⁻¹. All thermodynamicproperties are reported under standard conditions (1 bar and 298 K).

TABLE 3 ΔG⁰ _(reaction) |E⁰ _(cell)| Cathode reaction Possible anodereactions [kJ mol⁻¹] [V] Carbon dioxide → Water → Oxygen 257.20 1.33Carbon monoxide 2OH⁻ → H₂O + 0.5O₂ + 2e⁻ CO₂ + H₂O + 2e⁻ → CO + Overall:CO₂ → CO + 0.5O₂ 2OH⁻ Glycerol → Glyceraldehyde 97.48 0.51 C₃H₈O₃ + 2OH⁻→ C₃H₆O₃ + 2H₂O + 2e⁻ CO₂ + C₃H₈O₃ → CO + C₃H₆O₃ + H₂O Glycerol → Lacticacid 68.08 0.35 C₃H₈O₃ + 2OH⁻ → C₃H₆O₃ + 2H₂O + 2e⁻ Overall: CO₂ +C₃H₈O₃ → CO + C₃H₆O₃ + H₂O Glycerol → Formic acid 46.53 0.24 C₃H₈O₃ +8OH⁻ → 3HCOOH + 5H₂O + 8e⁻ Overall: CO₂ + 0.25C₃H₈O₃ → CO + 0.75HCOOH +0.25H₂O Glucose → Gluconic acid 6.20 0.03 C₆H₁₂O₆ + 2OH⁻ → C₆H₁₂O₇ +H₂O + 2e⁻ CO₂ + C₆H₁₂O₆ → CO + C₆H₁₂O₇ Methane → Methanol 141.10 0.73CH₄ + 2OH⁻ → CH₃OH + H₂O + 2e⁻ Overall: CO₂ + CH₄ → CO + CH₃OH Methane →Carbon monoxide 52.68 0.36 CH₄ + 6OH⁻ → 5H₂O + CO + 6e⁻ Overall:0.75CO₂ + 0.25CH₄ → CO + 0.5H₂O Carbon dioxide → Water → Oxygen 1331.401.15 Ethylene 2OH⁻ → H₂O + 0.5O₂ + 2e⁻ 2CO₂ + 8H₂O + 12e⁻ → Overall:2CO₂ + 2H₂O → C₂H₄ + 3O₂ C₂H₄ + 12OH⁻ Glycerol → Glyceraldehyde 373.080.32 C₃H₈O₃ + 2OH⁻ → C₃H₆O₃ + 2H₂O + 2e⁻ Overall: 2CO₂ + 6C₃H₈O₃ →C₂H₄ + 6C₃H₆O₃ + 4H₂O Glycerol → Lactic acid 196.68 0.17 C₃H₈O₃ + 2OH⁻ →C₃H₆O₃ + 2H₂O + 2e⁻ Overall: 2CO₂ + 6C₃H₈O₃ → C₂H₄ + 6C₃H₆O₃ + 4H₂OGlycerol → Formic acid 67.35 0.06 C₃H₈O₃ + 8OH⁻ → 3HCOOH + 5H₂O + 8e⁻Overall: 2CO₂ + 1.5C₃H₈O₃ + 0.5H₂O → C₂H₄ + 4.5HCOOH Glucose → Gluconicacid −174.60 0.15 C₆H₁₂O₆ + 2OH⁻ → C₆H₁₂O₇ + H₂O + 2e⁻ Overall: 2CO₂ +6C₆H₁₂O₆ + 2H₂O → C₂H₄ + 6C₆H₁₂O₇ Methane → Methanol 634.80 0.55 CH₄ +2OH⁻ → CH₃OH + H₂O + 2e⁻ Overall: 2CO₂ + 6CH₄ + 2H₂O → C₂H₄ + 6CH₃OHMethane → Carbon monoxide 209.60 0.18 CH₄ + 6OH⁻ → 5H₂O + CO + 6e⁻Overall: 2CO₂ + 2CH₄ → C₂H₄ + 2CO + 2H₂O

EXAMPLES Example 1. Electrochemical Performance for the Electroreductionof CO₂

Unless stated otherwise, all experiments were performed under ambientconditions of 1 atm and 293 K, all commercially available materials wereused as received, and >18.0 MO cm deionized (DI) water was used whenrequired.

The electrolytes used herein were prepared by dissolving the appropriateamount of the salt and/or chemical in DI water. The salts and chemicalsused were: potassium hydroxide (Fisher Chemical, product number: P250),glycerol (Alfa Aesar, product number: 38988), D-(+)-glucose (Sigma LifeScience, product number: 49139). The pH and conductivity of thedifferent electrolytes were measured using an Orion 4-star pHconductivity meter.

The electrochemical characterization of the different combinations ofCO₂ electroreduction at the cathode with the O₂ evolution reaction andglycerol, glucose, or CH₄ electrooxidation at the anode was performed ina gas diffusion electrode based dual electrolyte channel flowelectrolyzer with a precisely machined active geometric area of 1 cm²,as described previously. The catholyte and the anolyte chamber wasseparated by a Fumapem FAA-3-PK-75 anion exchange membrane to preventcrossover of the liquid products from the cathode to the anode and viceversa. The catholyte for all experiments was 2.0 M KOH. The anolyte forstudying the OER and CH₄ electrooxidation was 2.0 M KOH whereas theanolyte for studying the electrooxidation of glycerol and glucose was2.0 M KOH+2.0 M glycerol and 2.0 M KOH+2.0 M glucose, respectively.Electrochemical experiments were performed by maintaining a constantcell potential using a potentiostat (Autolab PGSTAT-30, EcoChemie). Theindividual cathode and anode potentials were measured with a multimeter(AMPROBE 15XP-B) connected between the appropriate electrode and anAg/AgCl reference electrode (3 mol kg⁻¹, RE-5B BASi). The individualelectrode potentials (vs. Ag/AgCl) were then converted to the RHE scaleusing the Nernst equation: E_(RHE)=E_(Ag/AgCl)+0.210+0.058×pH. All cell,cathode, and anode potentials are reported as measured without any iRcorrections. The CO₂ (Airgas) feed for the reaction was provided as acontinuous stream over the teflonized side of the cathode gas diffusionlayer (GDL) using a flow controller (Smart Trak 2, Sierra Instruments).A CO₂ flow rate of 17 sccm was maintained for cell potentials at whichthe total current density (j_(Total)) was >5 mA cm⁻² and lowered to 5sccm for cell potentials at which j_(Total) was <5 mA cm⁻², to enable agas product analysis with high sensitivity. A pressure controller (ColeParmer, 00268TC) was used in the electrolyzer downstream to maintain alow pressure of 14.20 psi and thus facilitate an easy transfer of thegas products from the cathode GDL to the effluent gas stream. A lowdownstream pressure also minimized the dissolution of the reacting CO₂and the gas products into the electrolyte stream. Both the catholyte andthe anolyte stream was circulated through the electrolyte channels ofthe electrolyzer using a syringe pump (PHD 2000, Harvard Apparatus) atflow rate of 0.5 mL min⁻¹ for cell potentials at which j_(Total) was >5mA cm⁻² and lowered to 0.2 mL min⁻¹ for cell potentials at whichj_(Total) was <5 mA cm⁻², to enable a liquid product analysis with highsensitivity. For all electrochemical experiments, after a particularcell potential was switched on, the resulting current was allowed tostabilize for at least 180 seconds before the product analysis wasinitiated.

For a particular cell potential, the gas products of CO₂electroreduction were analyzed for a total time period of 180 seconds bydiverting 1 mL of the effluent gas stream, thrice, at regular intervalsof 90 seconds to an on-line gas chromatograph (Thermo Finnigan Trace GCwith a Carboxen 1000 column from Supelco). The GC was equipped with boththe thermal conductivity detector (TCD) and the flame ionizationdetector (FID). Helium with a flow rate of 20 sccm was used as thecarrier gas. The concentration of the gas products was quantified byaveraging the peak areas over the three sample injections and using theappropriate calibration curves. Meanwhile, the liquid products wereanalyzed for the same 180 second time period by collecting both thecatholyte and the anolyte streams followed by ex situ ¹H NMR (UI500NB,Varian) analysis (16 scans with solvent suppression). The liquid samplesfor the ¹H NMR analysis were prepared by mixing 100 μL of the collectedelectrolyte with 400 μL of D₂O (Aldrich, product number: 151882) and 100μL of an internal standard comprising of 1.25 mM DMSO in D₂O. Theconcentration of the liquid products was quantified using theappropriate calibration curves. The total current density (=the totalcurrent as the electrolyzer area is 1 cm²) was quantified by averagingthe data obtained during the same 180 second time period when the CO₂electroreduction products were being analyzed. The Faradaic efficiencyfor the different CO₂ electroreduction products was calculated per thefollowing equation:

${{FE}\mspace{14mu} (\%)} = {\frac{znF}{Q} \times 100}$

where z is the number of electrons exchanged to form a particular CO₂electroreduction product, n is the number of moles of the productformed, F is the Faraday's constant (96485 C mol⁻¹), and Q is the amountof charge passed. The partial current density for a particular productwas calculated by multiplying j_(Total) with the Faradaic efficiency forthat product. The onset cell potential for a specific CO₂electroreduction product defined in this work refers to the lowest(least negative) cell potential at which the product is first observedin the GC (for gas products) or ¹H NMR analysis (for liquid products).

The cathode was a 1±0.1 mg cm⁻² Ag nanoparticle coated gas diffusionlayer (GDL) electrode. The anode was a 1±0.1 mg cm⁻² IrO² coated GDLelectrode for O₂ evolution, and a 1±0.1 mg cm⁻² Pt black coated GDLelectrode for glycerol and glucose electrooxidation. All data collectedunder ambient conditions of 1 atm and 293 K.

As indicated by the Gibb's free energy analysis, many different anodereactions other than OER can be utilized to lower |E⁰ _(cell)|, andhence the overall electricity requirements for CO₂ electroreduction. Toassess the practicality of such processes, we performed an experimentalelectroanalytical evaluation of the different combinations proposed inTable 3, using a gas diffusion layer (GDL) electrode based dualelectrolyte channel flow electrolyzer under ambient conditions. Thecatholyte was chosen as 2.0 M KOH, previously demonstrated by us tolower overpotentials and improve activity for CO₂ electroreduction. Theanolyte was chosen as a mixture of 2.0 M KOH and 2.0 M glycerol, amixture of 2.0 M KOH and 2.0 M glucose, and 2.0 M KOH for theelectrooxidation of glycerol, glucose, and CH₄ respectively.

The electrooxidation of glycerol or glucose on a Pt black coated GDLanode coupled to the electroreduction of CO₂ on a Ag coated GDL cathoderesulted in a significant lowering (i.e., less negative value) of theonset cell potential for CO formation, with a value of −0.75 and −0.95 Vbeing observed, respectively, in comparison to the state of the artvalue of −1.6 V with OER at the anode (FIG. 4A). However, the activity(partial current density for CO, j_(CO)) with glucose electrooxidation(j_(CO)=12.47 mA cm⁻² or production rate=0.065 kg_(CO) m⁻² h⁻¹ at a cellpotential of −1.5 V) was much lower than with glycerol electrooxidation(j_(CO)=88.44 mA cm⁻² or production rate=0.462 kg_(CO) m⁻² h⁻¹ at a cellpotential of −1.5 V) at the anode. These results indicate that theelectroreduction of CO₂ to CO could indeed become carbon neutral and/ornegative even when using the present-day grid electricity mix to drivethe process. Depending on the j_(CO) value, the electrooxidation ofglycerol at the anode instead of OER results in a 37 to 53% reduction inelectricity requirements, thus improving the process economics. Singleelectrode plot suggests the major improvement to be at the anode withthe cathodic CO₂ electroreduction remaining unaffected (FIG. 4B). Theanodic glycerol electrooxidation results in the formation of value-addedchemicals such as formate and lactate that further improves theeconomics of the overall process. Further, we also evaluated thedurability of CO₂-glycerol co-electrolysis with respect to CO production(FIG. 5). The results indicate the cell potential and FE_(CO) to befairly stable over a 1.5 h time period. However, flooding of theelectrolyte through the cathode GDL was observed at −1.5 h (similar toearlier observations in the literature) indicating the need to developmore durable GDLs to improve the prospects of this process.

A similar lowering in onset cell potentials for the electroreduction ofCO₂ to HCOO⁻, C₂H₄, and C₂H₅OH was observed when utilizing theelectrooxidation of glycerol at the anode instead of OER (FIG. 6A). Forexample, the onset cell potential for the electroreduction of CO₂ toHCOO⁻ on a Sn coated GDL cathode and C₂H₄, C₂H₅OH on a Cu coated GDLcathode was −0.9, −0.95, and −1.3 V, respectively, with the anodicelectrooxidation of glycerol, in comparison to −1.75, −1.8, and −2.1 Vwith the anodic OER (FIG. 6B and FIG. 6C). Preliminary experiments withthe electrooxidation of CH₄ on a Pt black, Cu, Pd, IrO₂, and Pt—Ru blackcoated GDL anode coupled to the electroreduction of CO₂ on a Ag coatedGDL cathode did not result in a change in the onset cell potentials forCO production, in comparison to OER at the anode. This is of courseexpected due to the high dissociation enthalpy of the C—H bond in CH₄(435 kJ mol⁻¹). For these experiments, the anode was a 1±0.1 mg cm⁻²IrO₂ coated GDL electrode for 02 evolution and 1±0.1 mg cm⁻² Pt blackcoated GDL electrode for glycerol electrooxidation. The catholyteincluded 2.0 M KOH. The anolyte included 2.0 M KOH for 02 evolution, and2.0 M KOH+2.0 M glycerol for glycerol electrooxidation. All datacollected under ambient conditions of 1 atm and 293 K.

In summary, we have shown that the prospects of CO₂ electroreduction, interms of both cradle-to-gate CO₂ emissions and economics can bedrastically improved by looking beyond the conventionally used OER atthe anode, which essentially acts as an energy sink. The indicate thatseveral different anodic reactions are available to replace the OER,thereby yielding superior thermodynamic processes with a lower |E⁰_(cell)|. Of the alternatives, the electrooxidation of glycerol (a cheapindustrial waste) seems particularly promising with the resultingprocess (co-electrolysis of CO₂ and glycerol) lowering the electricityrequirements for conventional CO₂ electroreduction approaches by up to53%. The new process offers avenues for integrating two different CO₂mitigation approaches i.e., CO₂ electroreduction and biodieselproduction as well. Furthermore, with the future development of moreactive and selective catalysts (particularly for glycerolelectrooxidation), co-electrolysis of CO₂ and glycerol can be improvedeven further, resulting in low energy pathways for the production ofcarbon chemicals from waste CO₂.

All of the methods disclosed and claimed herein can be made and executedwithout undue experimentation in light of the present disclosure. Whilethis invention may be embodied in many different forms, there aredescribed in detail herein specific preferred embodiments of theinvention. The present disclosure is an exemplification of theprinciples of the invention and is not intended to limit the inventionto the particular embodiments illustrated. In addition, unless expresslystated to the contrary, use of the term “a” is intended to include “atleast one” or “one or more.” For example, “a device” is intended toinclude “at least one device” or “one or more devices.”

Any ranges given either in absolute terms or in approximate terms areintended to encompass both, and any definitions used herein are intendedto be clarifying and not limiting. Notwithstanding that the numericalranges and parameters setting forth the broad scope of the invention areapproximations, the numerical values set forth in the specific examplesare reported as precisely as possible. Any numerical value, however,inherently contains certain errors necessarily resulting from thestandard deviation found in their respective testing measurements.Moreover, all ranges disclosed herein are to be understood to encompassany and all subranges (including all fractional and whole values)subsumed therein.

The invention encompasses any and all possible combinations of some orall of the various embodiments described herein. It should also beunderstood that various changes and modifications to the presentlypreferred embodiments described herein will be apparent to those skilledin the art. Such changes and modifications can be made without departingfrom the spirit and scope of the invention and without diminishing itsintended advantages. It is therefore intended that such changes andmodifications be covered by the appended claims.

Furthermore, the advantages described above are not necessarily the onlyadvantages of the invention, and it is not necessarily expected that allof the described advantages will be achieved with every embodiment ofthe invention.

1. A method of electroreducing carbon dioxide, comprising: feeding afirst stream comprising carbon dioxide into a chamber through a chamberinlet, the chamber containing a gas diffusion cathode and a gasdiffusion anode; feeding a second stream comprising glycerol or glucoseinto the chamber, the second stream having a pH of 12 to 14; andapplying an electrical potential between the gas diffusion anode and thegas diffusion cathode to reduce the carbon dioxide and oxidize theglycerol or glucose.
 2. The method of claim 1, further comprisingwithdrawing a liquid product stream from an outlet of the chamber, theliquid product stream comprising an oxidation product of glycerol orglucose or a reduction product of carbon dioxide and withdrawing agaseous product stream comprising a reduction product of carbon dioxideselected from carbon monoxide, ethylene, methane, and any combinationthereof.
 3. The method of claim 2, further comprising: a) purifying theliquid product stream to obtain a substantially pure stream of theoxidation product of glycerol or glucose or the reduction product ofcarbon dioxide; or b) purifying the gaseous product stream to obtain asubstantially pure stream of the reduction product of carbon dioxide. 4.The method of claim 1, wherein the gas diffusion cathode divides thechamber into a gaseous region and a liquid region.
 5. The method ofclaim 1, wherein the second stream comprises from 0.5 M to 2.5 M of analkali metal hydroxide.
 6. The method of claim 1, wherein the carbondioxide is reduced to carbon monoxide, and wherein a partial currentdensity for carbon monoxide is from 15 mA cm⁻² to 350 mA cm⁻² at a cellpotential of −1.0 V to −2.5 V.
 7. The method of claim 1, wherein thecarbon dioxide is reduced to a reduction product of carbon dioxide andan onset cell potential for formation of the reduction product of carbondioxide is from −1.5 V to −0.5 V.
 8. The method of claim 1, wherein thechamber is divided into a cathode compartment and an anode compartmentby an ion permeable membrane.
 9. The method of claim 8, wherein thesecond stream comprises an anolyte stream and a catholyte stream,wherein the anolyte stream is fed into the anode compartment and thecatholyte stream is fed into the cathode compartment.
 10. The method ofclaim 9, wherein the anolyte stream comprises an alkali metal hydroxideor an alkali metal carbonate.
 11. The method of claim 1, wherein the gasdiffusion cathode comprises an electrocatalytic cathode coatingconfigured for reducing carbon dioxide.
 12. The method of claim 11,wherein the electrocatalytic cathode coating comprises a metal selectedfrom silver, gold, zinc, palladium, tin, lead, mercury, indium, copper,and any combination thereof.
 13. The method of claim 1, wherein the gasdiffusion anode comprises an electrocatalytic anode coating configuredfor oxidizing glycerol.
 14. The method of claim 13, wherein theelectrocatalytic anode coating comprises a platinum black catalyst. 15.The method of claim 1, wherein the chamber further comprises an ionpermeable membrane in contact with the gas diffusion cathode.
 16. Themethod of claim 1, further comprising reducing the carbon dioxide toethanol, formate, or any combination thereof.
 17. The method of claim 1,wherein the chamber is defined inside a flow electrolyzer.
 18. Themethod of claim 1, wherein the pH of the second stream is 12.5 to 14.19. A method of electroreducing carbon dioxide, comprising: feeding agas stream comprising carbon dioxide into chamber divided into a cathodecompartment and an anode compartment by an ion permeable membrane, theanode compartment containing a gas diffusion anode and the cathodecompartment containing a gas diffusion cathode, the gas diffusioncathode dividing the cathode compartment into a gaseous region and aliquid region, wherein the gas stream is fed into the gaseous region;feeding a catholyte stream into the liquid region of the cathodecompartment; feeding an anolyte stream comprising glycerol into theanode compartment to contact the gas diffusion anode; and applying anelectrical potential between the gas diffusion anode and the gasdiffusion cathode to reduce the carbon dioxide to a reduction productand oxidize the glycerol to an oxidation product, wherein an onset cellpotential for formation of the reduction product is from −1.5 V to −0.5V.
 20. The method of claim 19, wherein the catholyte stream or theanolyte stream has a pH from 13 to 14.